The invention relates to a low-pressure mercury vapour discharge lamp comprising a glass discharge envelope in which a discharge is present during operation of the lamp and which contains mercury and a rare gas, at least a part of the inner surface wall of the discharge envelope being provided with a thin at least substantially homogeneous continuous transparent layer, which is resistant to the influence of the discharge.
It is known to take measures in low-pressure mercury vapour discharge lamps to prevent greying of parts of the inner wall of the discharge envelope which are in contact with the discharge. Such a greying, which is due to interaction of mercury and glass, is undersirable and not only gives rise to a reduction of the light output but also results in an unaesthetic appearance of the lamp, in particular due to the fact that the greying occurs irregularly, for example, in the form of dark stains and dots.
It is proposed in the U.S. Pat. No. 3,377,494 to provide the inner wall of the discharge envelope with a thin substantially homogeneous continuous transparent layer of, for example, titanium dioxide or zirconium dioxide in order to prevent greying of the glass inner wall of the discharge envelope.
In contrast to a protective granular layer consisting of a refractory metal oxide (such as aluminum oxide or silicon dioxide) which is composed of a large number of small particles and should be comparatively thick in order to prevent the occurrence of any interaction such a interaction between mercury and the glass wall. However in the lamp according to the aforementioned United States Patent Specification, a direct contact between the glass wall and the mercury discharge is avoided due to the presence of the thin homogeneous continuous transparent layer.
Due to the presence of the transparent layer, according to said Patent Specification, it is sufficient to use in the discharge envelope a comparatively thin luminescent layer, as a result of which a considerable saving in the required quantity of luminescent material is obtained compared with lamps not provided with a transparent protective layer.
It has been found that, especially in a transparent layer containing titanium dioxide, ultraviolet radiation having a wave length of approximately 350 nm is absorbed to a considerable extent; it has further been found that resonance radiation of mercury having a wave length of 354 nm is even absorbed to a substantially complete extent. This is especially disadvantageous when such a layer is employed in lamps which emit substantially exclusively radiation of the this wave length. Examples of such lamps are germicide lamps and lamps emitting ultraviolet radiation of comparatively long wave lengths, such as lamps intended for solarium arrangements.